Porous silicon (PS) suitable for optical detection of immunoreaction is fabricated. The structure of immunosensor is prepared by the following steps: oxidization, silanization, glutaraldehyde cross-linker, and covalent binding of antibody. When antigen is added into the immunosensor, the Raman intensity is estimated to be linearly reduced according to the concentration of the surface protective antigen protein A (spaA) of below 4.0 μg·ml?1. The ultimate detection limit is 1.412×10^2 pg.ml^{?1}. Controlled experiments are also presented with non-immune antigen of the spaA, and results show that the immunosensor has high specificity. Compared with the conventional enzyme-linked immuno sorbent assay (ELISA), this method is quick, inexpensive, and label-free.